Measure the induration, not the redness
Use a ruler in millimeters
Read the test 48 to 72 hours after placement
Record the exact size of the raised, firm area
0 to 4 mm: negative
5 to 9 mm: positive for some higher-risk groups
10 mm or more: positive for many risk groups
15 mm or more: positive for people with no known TB risk factors
Follow the cutoff used for your risk category
Seek medical interpretation if the result is unclear or you are high risk